if you go on priest you can try to w8 for him to cast 2+saving grace he will be with 30%+ reduced healing on top of dampening for 8 sec so his hp will drop for sure in this 8secs with all that -healing. Hoj/silence him when the grace stacks are about to expire /the moment when he will be out of the -healing debuff and he will bomb the big saving grace again/ this way you can maximize the pressure on him
but ye 2ble stun is the best option prolly

When I play my warrior which I think similar to dk, I just sit on the dps till the disc go out of mana, free win.
I think you could try keep attacking the dps, and take a swap on disc when hoj and silence is available.

I'm also around 2k on my DK (MW/DK), so take this with a grain of salt. My usual strat vs disc is to take the icy touch dispel glyph/ Remorseless Winter/chillblains. I open by pre-dispelling the priest as much as possible to clear all the buffs, then when I'm ready to burst, chains, close the gap, pop remorseless winter (and grip them back into it if they get away), then pillar of frost, then strang as the stun wears off, then kick whatever they try to cast next. If they get a shield up, I dispel it. If they live, they usually use desperate prayer, so I beat on them until the debuff is down, then switch to the dps until I have cooldowns again.

Usually can't kill them in one round of this, but once dampening get up and pain suppression is down it's not too bad. Rogues are the worst though, because they can peel you like mad. Against a hunter/feral if you pop AMS during burst, they have limited ways to peel you. Usually I have my MW partner sap the dps when I switch, and possibly stun.

No idea what really high level priests do. Dispel is RNG, but even if you get a few, it's better than eating the reflect, and dispelling the shield > the damage from the obliterate you lose. I'd love to hear why this would fail on higher level priests.